The invention relates to a process for operating a communication network of a motor vehicle, the communication network comprising a plurality of control devices which are connected to a data bus and can exchange messages with one another by way of this data bus. The plurality of control devices are connected to a voltage source. The control devices further have a voltage measuring device for determining the voltage applied to their supply connections. Each of the control devices can access, at least in a writing manner, a central and/or internal fault memory in order to document information therein concerning a fault detected by the concerned control device. The invention further relates to a control device for a communication network of a motor vehicle as well as to a communication network of a motor vehicle.
A fault memory entry documents the original cause of a function limitation occurring in the motor vehicle. This means that every fault memory entry is cause-related. A fault memory entry may be caused, for example, by a message received from another control device if the message itself or the content of the message does not correspond to the usual demands on the data format, the value range, etc. Fault memory entries are, for example, also generated when the voltage applied to a control device falls below a specified voltage threshold. This condition is called undervoltage.
For this purpose, each control device determines the voltage applied to its supply connection and checks whether there is an undervoltage. As a function of the determined voltage, each control device itself decides whether it can offer the functions provided by it. If this is not so, a failure will be documented in the fault memory. Missing messages from other control devices transmitted by way of the data bus are not documented in the fault memory in the event of undervoltage. Instead, only the fault memory entry representing the undervoltage will be kept available in the fault memory.
This leads to the problem that, when the motor vehicle is checked in a shop, the cause of the fault memory entry characterizing the undervoltage cannot be precisely recognized. In particular, it cannot be differentiated whether a defect is present at the control device, at a plug, at a cable, at a connection to a reference potential, at a communication partner or at an energy storage device of the motor vehicle. Expenditures and costs are therefore necessary in order to find the fault. This frequently has the result that the control device having the fault memory entry will be exchanged, which is connected with high costs.
It is an object of the invention to indicate a process which makes it possible to determine the cause of the fault in a simpler and more reliable fashion by means of a fault memory entry. It is a further object of the invention to indicate a corresponding computer program product, a control device as well as a communication network for a motor vehicle, which allow the implementation of the process.
These and other objects are achieved by a process according to the invention, a computer program product according to the invention, a control device according to the invention and a communication network according to the invention. Advantageous embodiments are described herein.
The invention provides a process for operating a communication network of a motor vehicle, the communication network comprising a plurality of control devices which are connected to a data bus and can exchange messages with one another by way of this data bus. The plurality of control devices are connected to a voltage source and have a voltage measuring device for determining the voltage applied to their supply connections. Furthermore, the plurality of control devices can access, at least in a writing manner, a central and/or internal fault memory in order to document information therein concerning a fault detected by the concerned control device.
The process comprises the following steps:                a) determining the global supply voltage of the voltage source;        b) checking whether the determined global supply voltage exceeds a specified upper voltage threshold or falls below a specified lower voltage threshold;        c) providing information representing the determined global supply voltage and/or the exceeding or falling-below to the control devices;        d) when a fault is detected by a respective control device, checking by use of the provided information whether the global supply voltage exceeds or falls below the respective voltage threshold;        e) when an exceeding or falling-below the concerned voltage threshold by the global supply voltage is present, a respective control device, when detecting a fault, instead of information representing the detected fault, will write into its fault memory information representing the exceeding or falling-below the concerned voltage threshold by the global supply voltage as a possible cause of the fault.        
The central distribution of the actual voltage situation of the voltage source (battery) of the motor vehicle makes it possible to better distinguish the cause of a fault memory entry. Particularly consequential faults of communication problems in the case of global undervoltage are explicitly identified by way of a fault memory entry characterizing the global undervoltage. This points in the direction of the battery voltage but does not result in repair instructions with respect to the control device. Only true faults at the control device, the plug or at the cable harness are attributed to a control device itself. This results in a simplified fault diagnosis. In addition, costs can be saved when the vehicles are repaired.
According to an advantageous further development, the determination of the supply voltage of the voltage source in Step a) and/or the checking whether the determined global supply voltage exceeds the specified upper voltage threshold or falls below the specified lower voltage threshold (Step b)) takes place in a central fashion. The providing of the information according to Step c) to the control devices takes place by way of a message transmitted by way of the data bus. The central detection of the global voltage constructively is not only simple but also makes it possible to measure the voltage with the greatest-possible precision. In particular, it is useful for the control device measuring the global voltage to be connected directly to the voltage source and/or to be arranged in the immediate spatial proximity of the voltage source.
It is further advantageous for Steps b) and c) to be carried out in cyclical intervals. This has the result that even when there is no exceeding of or falling below the threshold values, corresponding information is provided to the control devices. By way of the control devices receiving the information, when the information does not arrive, it can thereby be concluded that there is a defect of the central control device or the data bus communication. The implementation of Steps b) and c) at cyclical intervals thereby increases the certainty of the diagnosis of faults and the correctness of the resulting fault memory entries.
According to a further useful development, by way of a respective control device, when an internal fault caused by the control device is detected,                aa) it is determined before Step d) whether a determined local supply voltage exceeds a specified upper voltage threshold or falls below a specified lower voltage threshold;        bb) it is checked whether the condition in Step e) has been met;        cc) when it is determined that Step e) has not been met, when detecting the internal fault caused by the control device, instead of information representing the detected fault, information representing the exceeding or the falling below the concerned voltage threshold by the local supply voltage is written as a possible cause of the internal fault into the fault memory.        
This further development permits a differentiation between faults which could have been caused by a global or local over- or undervoltage. This further improves the precision of the fault diagnosis.
In a further advantageous development, the providing of information representing the determined global supply voltage and/or the exceeding or falling-below to the control devices takes place by use of a message transmitted by way of the data bus in a bit string, in which the setting of a flag at a specified point of the bit string signals the exceeding or falling below the concerned voltage threshold by the global voltage supply.
The providing of the determined global supply voltage should be equated with the analog transmission of the measured voltage. This has the result that Step b) can then be carried out by the control devices themselves. This means concretely that the checking whether the determined global supply voltage exceeds a specified upper voltage threshold or falls below a specified lower voltage threshold is carried out by the control devices.
In contrast, the transmission of the information to the control devices in a bit string has the advantage that the evaluation of the exceeding of or falling-below takes place by the control device carrying out the measurement. In the case of a change of the voltage thresholds, it is therefore sufficient to provide this change only in the control device carrying out the voltage measurement. The latter variant therefore offers administrative advantages.
The invention further provides a computer program product which can be loaded directly into the internal memory of a digital control device and comprises software code sections by which the following steps can be carried out when the product is running on the control device of a communication network of a motor vehicle.                a) receiving information representing a centrally determined global supply voltage and/or an exceeding of a specified upper voltage threshold or a falling-below a specified lower voltage threshold;        b) when a fault is detected by the control device, a checking by use of the provided information is carried out as to whether the global supply voltage exceeds or falls below the respective voltage threshold;        c) when an exceeding or falling-below the respective voltage threshold by the global supply voltage is present, the control device, when detecting a fault, instead of information representing the detected fault, will write into its fault memory information representing the exceeding or falling-below the respective voltage threshold by the global supply voltage as a possible cause of the fault.        
In a further advantageous embodiment, the computer program product is further designed for implementing further steps of the above-described process.
The invention further provides a control device for a communication network, the communication network comprising a plurality of control devices which are connected to a data bus and can exchange messages with one another by way of this data bus. The control device is connected to a voltage source and has a voltage measuring device for determining the voltage applied to its supply connections. The control devices can access, at least in a writing manner, in each case, a central and/or internal fault memory in order to document information therein concerning a fault detected by the respective control device. The control device is designed for implementing the following steps:                a) receiving information representing a centrally determined global supply voltage and/or an exceeding of a specified upper voltage threshold or a falling-below a specified lower voltage threshold;        b) when a fault is detected by the control device, to carry out a checking by use of the provided information as to whether the global supply voltage exceeds or falls below the respective voltage threshold;        c) when an exceeding or falling-below the respective voltage threshold by the global supply voltage is present, the control device, when detecting a fault, instead of information representing the detected fault, is to write into its fault memory information representing the exceeding or falling-below the respective voltage threshold by the global supply voltage as a possible cause of the fault.        
In a further advantageous embodiment, the control device is further designed for implementing further steps of the above-described process.
Finally, the invention provides a communication network of a motor vehicle, the communication network comprising a plurality of control devices which are connected to a data bus and can exchange messages with one another by way of this data bus, the plurality of control devices being designed in the above-described fashion.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.